1. Field of the Invention
The invention relates to a process and an apparatus for isolating flexible flat objects, particularly paper sheets, cardboard sheets, metal sheets or the like, whereby the objects are lifted upwardly off a stack, and are thereby removed individually and successively in horizontal or approximately horizontal transport direction.
2. Description of Related Art
It is known in the art to lift flexible flat objects off a stack, e.g. metal sheets, by their full area using a vacuum device, and then to transport them horizontally in a transport direction to a further processing step. For precise further processing, these flat objects must be fed in an exactly defined set position so that subsequent processing steps, such as printing, stamping, etc. on the flat object take place at exactly reproducible points, which are defined, for example, with reference to one of the edges of the object.
Particularly if stacks are heavy and bulky, it may be difficult to position these stacks in relation to an isolation apparatus in a precisely reproducible manner. Moreover, any variation during lifting of the individual objects off a stack can have the result that objects that have been lifted off the stack are removed and fed to a subsequent processing unit in irregular positions with respect to each other.
DE 19642484 A1 discloses a printing machine with a rotating isolation device in which a rotary table is arranged above a sheet stack and serves to rotate the top sheet of the stack and thereby causes the second sheet to be isolated and exposed. An alignment unit, with the aid of friction forces, orients the top sheet of the stack into a set position.
To be effective, this apparatus requires that the friction forces exerted by the alignment unit on the sheet surface are greater than the adhesion forces that hold the top sheet to the stack. This requirement cannot be met for all the materials of which the flat objects to be isolated can be made.
Although it is possible in an apparatus or process for isolating flat flexible objects chronologically and/or technically to separate the steps of lifting an object from the stack and orienting this object, it would prolong the isolation process and substantially complicate and increase the required apparatus.
The goal of the invention is to define a process and an apparatus of the generic class, which will make it possible simply and quickly to align flat objects to be isolated into a set position before they are removed for further processing.
According to a first aspect of the invention, this object is attained by a process as defined in claim 1. By integrating the operation of aligning the object into a set position with that of lifting the object off the stack, any delay due to alignment is avoided. Furthermore, alignment of the object takes place at a stage where the adhesion forces between the object and the stack are largely overcome. This eliminates the risk that the means used to align the object and engage therewith do not slide off ineffectively.
This goal is preferably attained in that the object is first lifted along one of its edges, then aligned, and finally lifted off the stack over its entire extent.
Depending on which edge of the object is to serve as reference in further processing, the position of the lifted edge or that of the opposite edge can be adjusted to a set position to align the object. To align the object, the edge serving as reference can be moved via sensors that are remote from the stack either in the direction or against the direction of transport, in order to detect the orientation of the edge and possibly to shift the edge in a controlled manner and thus to align it based on the measurement result of the sensor. Alternatively, the edge can be brought into contact with at least one stop element that defines the set position.
If the rigidity of the flexible object is low, alignment along a set position preferably comprises pulling the object in the direction of its lifted edge. Since this edge is free above and below, it can be clamped if necessary to exert a substantial pull thereon and thus reliably to overcome any residual adhesion to the stack of the object area that is still resting on the stack.
If the rigidity of the material is great enough and/or the residual adhesion to the stack of the partially lifted object is low, alignment to a set position preferably comprises pushing the object in the direction of its non-lifted edge. This movement frees the edge to be lifted of the next object to be isolated from the stack, which can then be lifted even before the previous object has been completely aligned or removed.
In addition to aligning the lifted edge or the edge opposite thereto, it is advantageous also to adjust a lateral edge of the object to a set position. This completely defines the position of an object entering a further processing unit both in the direction of transport and perpendicularly thereto.
Alignment is advantageously effected by displacing and/or rotating the lifting device, which thus simultaneously acts as an alignment device.
A lifted object is preferably removed with the initially lifted edge pointing to the rear. This makes it possible to lift the next following object in the stack by its edge even before the previously lifted object has been lifted over its entire extent or removed.
To prevent sagging of the object along the lifted edge, this edge is preferably held undulately curved about a horizontal axis as it is being lifted.
To prevent uncontrolled lateral offset of the object during lifting, it has been found to be advantageous to define a segment of the object edge to be lifted first where the lifting process is to start. This segment can be a central segment or a comer of the object. In the course of continued lifting of the edge, adjacent areas should then also be lifted until the entire edge is on a predefined level.
According to a second aspect of the invention, the goals is attained by an apparatus for isolating flexible flat objects as claimed in claim 17. The apparatus comprises a lifting device that lifts the object along one of its edges and aligns it into a set position in its partially lifted state. In this state, the edge of the object opposite the lifted edge can still rest on top of the stack. Since adhesion between the stack and the object is substantially reduced, however, the object can already be precisely aligned in this state.
A preferred embodiment provides that the transport device of the apparatus according to the invention comprises a holding device that separates the partially lifted object over its entire extent from the stack and keeps it separate, and that comprises means to drive the held object in transport direction in order to feed it possibly to a further processing unit.
The apparatus preferably comprises at least one alignment element serving as reference to define a set position of an edge of the object. This edge can be the lifted edge itself or an edge opposite thereto, depending on the requirements of the subsequent processing unit. The alignment element can be a stop element with which the edge of the object is brought into contact. As an alternative or in addition thereto, a sensor can be used to detect the position of an edge of the object and to supply a measure for the movement necessary to align the object correctly. This movement can be a translation and/or a rotation of the object about a vertical axis for which the object is preferably driven by the lifting device.
According to a preferred embodiment, the lifting device comprises a suction head arrangement or a suction box extending in the direction of the lifted edge. Such an arrangement or suction box holds the flexible object over substantially the entire length of its edge. A suction box with a straight underside thus prevents the edge from sagging.
Such a linear course of the edge facilitates a precise positioning of the object crosswise to the transport direction. However, the underside can also be curved, so that it also curves the object lying against the suction box. As a result, the object is stiffened parallel to the transport direction to facilitate its precise positioning in this direction.
According to a preferred embodiment, the apparatus comprises both the arrangement of suction heads and the suction box, with the functions of lifting the edge and aligning the object being divided between the two elements.
To prevent the top side of a completely lifted object from scratching along the suction box during subsequent removal, a preferred embodiment provides that the apparatus blow air through the suction box when a lifted object is removed, in order to keep the object at a distance from the suction box during removal.
The holding device of the apparatus according to the invention is preferably arranged above the stack such that it exerts a force of attraction in downward direction, and the lifting device lifts the edge of the object far enough so that it is caught by this force of attraction. At first, therefore, an area of the object adjacent to the lifted edge reaches the range of action of the holding device and is lifted by it. Lifting and adhesion to the holding device thus propagates from the initially lifted edge over the entire length of the object until the object is completely lifted off the stack. In this state, the object can be removed and possibly fed to a further processing unit.
A preferred embodiment of the invention provides that the lifting device hold the lifted edge of the object undulately curved about an axis parallel to the edge. Such a curve serves to stiffen the partially lifted object in the direction parallel to the edge. This curve can be produced, for example, by providing the suction heads or the suction box of the lifting device with a contact surface for the object that slopes downwardly in the direction toward the edge, or by tilting a contact surface, which is horizontally oriented when an object on the stack is pulled up by suction, about an axis parallel to the edge during lifting.
Actuators for horizontal displacement of the object held to the holding device into the alignment position preferably comprise, respectively, a housing, a shaft displaceable within the housing in one or two spatial directions, and expandable elements disposed in pairs opposite each other in an annular space between the housing and the shaft, which can be pressurized with a pressure medium to adjust the position of the shaft in relation to the housing in the spatial direction or directions.
This shaft can be hollow and in turn can form part of a suction air channel of the lifting device. In this case, the expandable elements can perform not only the function of adjusting the shaft but can also seal the suction air channel against the environment.
To achieve a high degree of mobility of the shaft in relation to the housing, the expandable elements are preferably provided with folded sidewalls to permit strong expansion or compression of the expandable elements in the direction to be adjusted.
The pressure medium pressurizing the expandable elements is preferably air.
Substantially play-free guidance of the shaft in relation to the housing in a third spatial direction is achieved with the aid of parallel flanges formed on housing and shaft and located opposite each other in pairs. To be able to displace these flanges with little friction in relation to each other under the action of the expandable elements, they are advantageously spaced at a distance from each other by rolling elements. These rolling elements can be cylindrical or, in the preferred case where the position of the shaft can be adjusted in relation to the housing in two spatial directions, spherical.